An alternative to combat air pollution resulting from the combustion of fossil fuels is to remove the pollutants prior to combustion and thus burn a clean fuel. The problem is to produce a "clean" fuel, i.e., a fuel containing low sulfur, low nitrogen, and low ash values. This invention relates to a new and improved process for producing such a clean fuel.
A primary object of the present invention is to provide a process for producing a clean coal which can be used as fuel with minimal pollution control and as a raw material for the production of gaseous and liquid fuels by gasification and liquefaction.
This invention can enhance the usefulness of coal as a contributor to the energy requirements of the United States and greatly reduce the pollution of our environment. The clean coal can be not only a source of solid fuel but also a raw material for the production of gaseous and liquid fuels. Pollution of the environment with sulfur and nitrogen oxides and toxic metals from the combustion of coal can be greatly reduced as a result of their removal from the coal prior to combustion. Furthermore this invention will alleviate the dependence of the United States on foreign sources of energy and also alleviate the energy crisis the United States is now facing.
Coal is the major source of energy for the United States and will continue to be for many years. However, one of the problems with coal as the source of energy is its high sulfur, nitrogen, and ash content which includes significant quantities of toxic (hazardous) impurities such as mercury, beryllium, and arsenic. These materials find their way into the environment during the combustion of coal and, thus, constitute a health hazard through atmospheric and food chain comsumption.
Physical separation of these constituents from coal is not satisfactory as, at best, only a portion of them are removed. Furthermore, flue gas scrubbing is not entirely satisfactory as a means for sulfur and hazardous metals removal as at the present stage of development such systems (primarily for sulfur emissions control) are about 75 percent efficient, large quantities of sludges are formed which present a disposal problem, and the cost for flue gas scrubbing is high. Since the quantity of low-sulfur coal is limited and coal is our major source of energy, new or improved technology must be developed for cleaning coal prior to combustion to supply the U.S. with a clean coal and, at the same time, reduce the pollution of our environment. This clean coal can be used directly as a solid fuel and as a raw material for producing gaseous and liquid fuels.
The problem associated with producing a clean coal can be solved by employing chemical separation processing. Our research has shown that the majority of the sulfur and the majority of the ash including such toxic or hazardous metals as beryllium, boron, and lead can be extracted directly from the coal by a hydrothermal process
The three different classes of impurities--sulfur, nitrogen, and metal values--are found in coal in a variety of ways.
Sulfur occurs in coal chiefly in three forms: (1) inorganic, (2) sulfate, and (3) organic. A fourth form, elemental sulfur, is rare. Of the inorganic sulfur compounds, iron pyrite (FeS.sub.2 with an isometric crystal form) and marcasite (FeS.sub.2 with the orthorhombic crystal form) are the most common. Other inorganic sulfides, chalcopyrite -- CuFeS.sub.2, arsenopyrite -- FeAsS, and stibnite -- Sb.sub.2 S.sub.3, have been found, but they are rare.
Of the two major inorganic sulfides pyrite is the most common. It is found in coal as macroscopic and microscopic particles as discrete grains, cavity fillings, fiberbundles and aggregates. The concentrations of pyritic sulfur vary widely even within the same deposit. Normally, the concentration will vary from 0.2 to 3 percent (sulfur basis), depending on the location.
The most common sulfate sulfur is calcium sulfate. Sulfates or iron, copper, and magnesium may also occur, but they are not abundant. Normally coal contains less than 0.1 percent sulfate sulfur, although in heavily weathered coal, it may be such as 1 percent. Because of its normally low concentration, it is of little concern in air pollution.
The third form of sulfur most prevalent in coal is organic sulfur. Since this sulfur is part of and is linked to the coal itself, positive identification of the organic sulfur compounds has not been possible. However, it is usually assumed that organic sulfur is in one of the following forms:
1. Mercaptan of thiol, RSH PA1 2. sulfide or thio-ether RSR' PA1 3. disulfide, RSSR' PA1 4. aromatic systems containing the thiophene ring.
The sulfur could be present as .delta. -- thiopyrone.
No definite relationship between the organic and pyritic sulfur contents of coal has been established. In typical U.S. coal, the organic sulfur has ranged from 20.8 to 83.6 percent of total sulfur and has had no mean value of 51.2 percent of the total sulfur. The variation of the organic sulfur content of a coal bed from top to bottom is usually small. Pyritic sulfur may vary greatly.
Nitrogen, like sulfur, is probably part of and linked to the coal. Eastern coals average about 1.4 percent nitrogen, but with a range of 0.7 to 2.5 percent.
Metal values make up the part of coal commonly referred to as ash. They are found in coal as macroscopic and microscopic particles as discrete particles, cavity fillings, and aggregates. Concentration ranges from a few percent to 15 or 20 percent.
In addition to being a source of particulate emissions to the atmosphere, ash can give rise to operating problems. For example, excessive spalling and fluxing of refractories have been associated with the presence of sodium in the fuel. Quality of product may be adversely affected by ash from the flame, while build-up of deposits of ash on furnace tubes can reduce the rate of heat transfer. In addition, some types of deposits in the molten state may cause severe corrosion.